Two years ago the collaboration of Event Horizon Telescope (EHT) excited the world when it released the very first image of hot plasma swirling around the event horizon of the supermassive black hole in the center of a galaxy, Messier 87. Now the EHT- collaboration has reported the results to measure the polarization of the millimeter-wave radiation emitted by the plasma – both with EHT and the Atacama Large Millimeter Array (ALMA) in Chile.
The resolving power of a telescope is proportional to the observation wavelength divided by the diameter of the telescope. In the case of the EHT, whose constituent telescopes encompass the globe to form a giant array, its operating wavelength of 1.3 mm gives an angular resolution of 20 microseconds. This is equivalent to solving a golf ball on the surface of the Moon, and it is enough to visualize a supermassive black hole in a not too distant galaxy. The telescopes that make up ALMA are a maximum of a few kilometers away. They cannot solve extragalactic black holes. But when they start using M87, they provide complementary data.
Magnetic fields penetrate the plasma, which envelops black holes. As the electrons of the plasma revolve around the field lines, they lose energy by synchrotron radiation. The radiation is linearly polarized in the plane of rotation. As the radiation passes through the plasma along the observer’s line of sight, another electromagnetic process, Faraday rotation, takes place. If all the magnetic field lines around M87 were neatly aligned, the polarization of the radiation would be strong and rotated by the same amount. But if instead the field lines were tangled a mess, any polarization originally had the radiation would be exterminated.
As solved and mapped by EHT, the fractional polarization of the radiation of the M87 event horizon is at most 20%. ALMA’s observations, which average the polarization over a larger region, give an upper limit of about 3%. The observed pattern corresponds to a poloid magnetic field – that is, a field with north and south poles like the Earth’s. Such a field could be generated by a plasma orbiting around the black hole in an overgrown disk.
In particular, the observed polarization is consistent with what astrophysicists call a scenario magnetically arrested disk (MAD). As the magnetic field grows near the event horizon, it becomes so intense that it bounces back against the plasma drawn into the black hole. The scenario forces the magnetic fields to be oriented perpendicular to the overhead disk (along the north and south poles of the rotating black hole), and this is what gives the polarized image the “twist”.
M87 is a giant elliptical galaxy of trillions of stars in the center of the Virgin supercluster. A jet of hot plasma emerges with relativistic velocities from the galaxy’s core (see accompanying image). In 1977 Roger Blandford and Roman Znajek proposed a way in which a rotating black hole could extract energy from surrounding plasma and transform it into a jet. Can the Blandford-Znajek mechanism be responsible for the M87 jet, given the values for field strength, temperature, and density inferred from the EHT and ALMA observations?
To find out, the collaboration created a set of jet models and then simulated how their related radiation would appear if observed by EHT at the distance of M87-4900 light-years. The model that most closely matched the observations validated an assumption made by Blandford and Znajek: that magnetic fields in the vicinity of the event horizon are dynamically important for jet production. (K. Akiyama et al., Collaboration of Event Horizon Telescope, Astrophys. J. Lett. 910, L13, 2021.)